(1) Field of the Invention
The present invention relates to an apparatus, and method for using such an apparatus, for controlling the flowpath area in a gas turbine engine. More specifically, the present invention relates to an apparatus for providing non-linear flowpath area control of a gas turbine engine through the use of vernier duct blocker.
(2) Description of the Related Art
When operating gas turbine engines, it is frequently desirable to control the amount of gas flowing through the secondary flowpath between the outer duct and the inner support structure. One common method of achieving such control is to install an apparatus for adjusting the area through which the gas may flow. Such flow blockers often include a rotatable member with a plurality of openings that can be rotated in order to control the size of an open area through which gas can flow. With reference to FIG. 1, there is illustrated one such apparatus. A number of vanes 21 are circumferentially attached about a ring or rings each vane separated from its neighbor by a uniform distance w. Each vane is formed of a forward portion 15 and an aft portion 13, which, together, form an airfoil shaped vane 21. Between each forward portion 15 and each aft portion 13, there is positioned a rotatably movable ring 11. Rotatably movable ring 11 has a series of openings 17 each of a width w and spaced so as to generally correspond to the widths w between each adjacent pair of vanes 21.
With reference to FIG. 4a there is illustrated a view of a portion of a flow blocker 10 looking aft and directly towards the leading edges 23 of the aft portions 13. To assist in visualization, the forward portions 15 of each vane 21 are not shown. When the openings 17 of the rotatably movable ring 11 are aligned with the spaces between adjacent vanes 21, the flow blocker 10 is in a fully open position whereby a maximum opening, consisting of the sum of all unblocked openings 17, is created. With reference to FIG. 4b, it is evident that when rotatably movable plate 11 is rotated, the sum of the unblocked portions of all openings 17, is substantially reduced.
It is most desirable for a flow blocker 10 to provide for complete, or nearly complete, blockage of gas flow when necessary while causing little if any blockage when needed. In addition, it is often the case that there are located several flow blockers arranged in series along a central axis 19 of a gas turbine engine. Depending on the flight envelope in which an engine is operating, differing flow blockers will be adjusted to provide for differing opening areas through which gas can flow. Unfortunately, there typically exists a linear relationship between the angular rotation of the rotatably movable ring 11 and the size of the resultant opening through which gas can flow. As a result, in instances wherein one wishes to finely control the area of an opening such that only a small area is provided through which gas can flow, small angular adjustments of the rotatably movable ring 11 result in relatively large differences in the opening area through which gas can flow.
What is therefore needed is a flow blocker 10, and method for so using such a flow blocker, that permits a wide range of adjustable opening sizes through which gas can flow while allowing for fine control of the opening sizes when a small opening size is desired.